A number of image processing systems to prevent the illegal duplication of banknotes and stock certificates have been proposed and implemented in the prior art. These approaches involve installing a device in a system which is closed from the input of the original image into the color copier until the reproduced image is generated. In this sort of closed system, the image input unit reads the original (a banknote or other document which may not legally be copied) and uses the data to verify that it may not be copied. Based on this result, output is prohibited in the image generating device.
In recent years, as image scanners, personal computers and printers have become more sophisticated and cheaper, illegal copying in open systems has become more of a social problem. An image scanner can be used to read a banknote, and the image data which it reads can be stored in a personal computer. The data can be sent from the computer to a color printer, which can then generate and print out the image. In such a case, the image data stored in the personal computer may have been sent directly in the form of signals from a scanner, or they may have been acquired off a recording medium such as a floppy disk or a magneto-optical disk. In this sort of open system, then, images are often generated from data of unknown origin. In these cases, too, some means must be provided to prevent the output of prohibited images.
The problem which arises in such cases is that the image data which arrive at the printer's image generator are not necessarily identical to those read from the original image. At the point when a decision must be made in the image generator as to whether the image it is about to output is or is not one which may not be copied, key features of the image may have already been lost.
In particular, in order to discourage tampering, the characteristic mark printed on money or stock certificates to show that they may not be copied is generally made so small that it is nearly imperceptible to the eye. If the computer processes the image to modify it slightly, there is a high probability that some of the data will be lost.
Some have proposed that the image be verified as legally reproducible in the image scanner, so that the image will not even be read if it may not be copied. However, many feel that prohibiting even the reading of the image is excessive. It is extremely difficult to come up with an absolute way to prohibit all illegal copying.
The present invention was developed in consideration of the background described above.